This invention is directed to a switch mode power supply of the type described in EP 0 336 725 A2. With such a switch mode power supply, the amplitude of the operating voltage which is generated on the secondary side of a transformer, is stabilized by regulating the duty cycle of a switching transistor which is located on the primary side of the transformer. This control requires the transmission of a data signal, which is indicative of the amplitude of the operating voltage, from the secondary of the transformer to the primary of the transformer. However, this transmission, must not interfere with the electrical separation between the primary and the secondary sides of the transformer. Typically, in the prior art of the transmission of a data signal from the secondary to the primary of a transformer requires additional circuitry, such as a separate transmitter or an optocoupler.